Potamogeton robbinsii
Potamogeton natans
fern pondweed, potamot de Robbins, Robbin's pondweed
broad-leaf pondweed, floating pondweed, floating-leaf pondweed, potamot flottat
present.
present.
terete, without spots, to 100 cm;
glands absent.
terete, often rust-spotted, 30–90 cm;
nodal glands absent.
absent.
absent.
submersed, conspicuously 2-ranked, sessile, stiffish;
stipules persistent, conspicuous, adnate to base of blade about ± ¼ length of stipule, connate, greenish brown to white, ligulate, 0.5–2 cm, fibrous, shredding at tip, apex obtuse;
blade dark green to reddish green, linear to lanceolate, not arcuate, 2–7(–12) cm × 3–4(–8) mm, base rounded, with basal lobes, not clasping, margins minutely spinulose to serrulate, not crispate, apex not hoodlike, acute, lacunae absent;
veins 20–60, fine.
both submersed and floating, ± spirally arranged.
petioles lighter green immediately proximal to apex, 5.5–29 cm;
blade adaxially light green, elliptic to ovate, 3.5–11 cm × 15–60 mm, base cordate, apex acute to rounded;
veins 17–37.
sessile, rigid;
stipules persistent, conspicuous, convolute, free from blade, whitish, not ligulate, 4.5–1 cm, fibrous, not shredding at tip, apex obtuse;
blade light to dark green, phyllodial, not arcuate, 9–20 cm × 0.7–2.5 mm, base slightly tapering, without basal lobes, not clasping, margins entire, not crispate, apex not hoodlike, obtuse, lacunae absent;
veins 3–5, obscure.
often branched;
peduncles not dimorphic, axillary, erect, cylindric, 3–5(–7) cm;
spikes not dimorphic, moniliform (i.e., beaded), 7–20 mm.
unbranched, emersed;
peduncles not dimorphic, terminal, erect to ascending, cylindric, 4.5–9.5 cm;
spikes not dimorphic, cylindric, 25–50 mm.
stipitate, brown, obliquely obovoid, turgid, abaxially and laterally keeled, 3–4(–5) × 2(–3.3) mm, lateral keels without points;
beak erect, recurved at apex, 0.7–0.9 mm;
sides without basal tubercles;
embryo with less than 1 full spiral.
sessile, green to greenish brown, obovoid, turgid, not abaxially or laterally keeled, 3.5–5 × 2–3 mm;
beak erect to apically recurved, 0.4–0.8 mm;
sides without basal tubercles;
embryo with 1 full spiral.
= 52.
= 52.
Potamogeton robbinsii
Potamogeton natans
Potamogeton robbinsii is our most easily recognized species when it is fertile. It is the only species with branched inflorescences. The species, however, occurs in fairly deep water, forming large colonies that essentially cover the substrate. Only rarely do the plants flower. It also is the only species with truly auriculate leaves, the blades forming small lobes projecting past the stem on each side of the stem. Leaf blades of other Potamogeton species may have slightly rounded bases, but no others have lobes that actually protrude past the stem.
The species has a fairly large disjunction; primarily known from the northern part of the flora, it also occurs in the Tensas River area, Baldwin County, Alabama. The Alabama population has been collected on at least two occasions over 40 years, once as recently as 1970.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Potamogeton natans is the common floating-leaved pondweed of the north temperate ure areas. It is essentially circumboreal and can easily be identified by floating leaves that are almost always cordate at the base of the blade, the petiole with a short band of light tissue at its apex, and the submersed phyllodial leaves. Also, the apex of the petiole usually is bent so that the blade appears oriented in the opposite direction from which the petiole appears to be oriented.
One hybrid, Potamogeton natans × P. nodosus (= P. × schreberi Fischer [P. × perplexus A. Bennett]), has been described.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
- Local floras:
BC, 
CA, 
OR, 
WA - Local Web sites: CalFlora, CalPhotos,
Flora NW, 
Go Botany, 
IL Wildflowers, 
MI Flora, 
MN Wildflowers, 
PNW Herbaria 
WildflowerSearch
iNaturalist (observations)
USDA Plants Database- LBJ Wildflower Center
- SEINet
- Plants of the World Online
- Encyclopedia of Life
- Wikipedia
- Google Image Search
- Local floras: BC, CA, OR, WA
- Local Web sites: CalFlora, CalPhotos, Flora NW, Go Botany, IL Wildflowers,
KS Wildflowers, MI Flora, MN Wildflowers, PNW Herbaria - WildflowerSearch
- iNaturalist (observations)
- USDA Plants Database
- LBJ Wildflower Center
- SEINet
- Plants of the World Online
- Encyclopedia of Life
- Wikipedia
- Google Image Search
